Laser Doppler Velocimetry (LDV) techniques have been utilized for many years to monitor the motion of moving objects. The coherence of the laser is utilized to project a “fringe pattern” on to the moving object. Small imperfections on the object such as scratches traverse the projected fringe pattern giving rise to an AC modulation in the reflected optical energy.
FIG. 1 shows the basic components of a single component LDV system commonly used in the art. The laser beam is split into two equal intensity beams and focused to an overlap region, which partially defines the sample volume. In order to obtain velocity measurements of an object, the object must possess some surface irregularity, which is analogous to particles in a flow for measuring the velocity of the flow. The surface irregularities on the object that pass through the sample volume scatter light, a part of which enters the receiver lens aperture. This light is focused onto the small aperture on the photodetector. If the receiver is located off-axis, the intersection of the image of the aperture and the focused laser beam form a very small sample volume. This technique can be used to effectively limit the detected light from only those particles that cross the overlap region of the beams.
Within the beam intersection region, light from the two incident beams interferes to create a fringe pattern. These fringes form parallel planes, which lie perpendicular to the plane of the incident beams yet parallel to the beam bisector. The spacing, delta, between successive fringe planes is given by   δ  =      λ          2      ⁢              sin        ⁡                  (                      θ            2                    )                    where λ is the light wavelength and θ is the beam intersection angle. Irregularities transit the probe volume; they scatter light in proportion to the light incident upon them, which varies with the intensity of the fringe pattern. It is very important to have a large degree of beam overlap to insure that the highest fringe visibility occurs.
Photomultiplier tubes are used to convert the detected light to an electronic signal. A familiar Doppler “burst” signal is shown in FIG. 2. The Doppler “burst” signals consist of a pedestal component which is a result of the Gaussian intensity distribution of the laser beam and a Doppler component. The Doppler or high frequency components arises from the irregularities passing the interference fringes. In addition to these frequency components there is the inevitable noise produced by such phenomena as distortions to the laser beams, glare light from optical components, and electronic noise.
Because of the inevitable noise present in the signal, signal processing must be utilized. With the most commonly used signal processors, the pedestal or low frequency component of the signal must be removed by a high pass filter. The high pass filter cutoff must be low enough to avoid excessive attenuation of the Doppler frequency information. However, if the filter is set too low the pedestal will not be fully removed producing a filter distortion. If the filter distortion is large enough, for example, due to a large signal, the frequency at the trailing part of the signal can be interpreted incorrectly by as much as an order of magnitude.
Several methods are available for processing LDV signals one of which is counter processors. Signal processing systems that produce information in the frequency domain are dependent on the transit time of the Doppler burst and are in general sensitive to the amplitude of the signal. Counter processors obtain measurements in the time domain, which are free from transit time broadening errors.
The basic principles of counting methods are relatively straightforward. High pass filtering is first required to remove the pedestal. If this is done properly, the zero crossings will be independent of the signal amplitude and represent the period of the Doppler difference frequency. Typically, the electronic system will average the period over a number of cycles in the burst, preferably all of the cycles that are above the threshold. However, measurements in the time-domain are independent of the burst duration so the number of cycles in the burst does not influence the measurements. With noise on the signals, averaging over the greatest number of cycles available can ameliorate the effects of any erroneous counts.
A signal is detected when the signal voltage exceeds a threshold level, which is set just above the baseline noise level. When the threshold is exceeded, three counters are started on the next zero crossing. Two counters measure the Doppler period and one counts the number of cycles.
A high-speed crystal stabilized clock (100 MHz to 1 GHz) is enabled and turned off at the end of N counts. The gate can open at any time so the count error can be as much as +/−1 clock count. Because high-speed counters are now used, this error is generally insignificant. As an example, if the Doppler difference frequency is 20 MHz and the clock frequency is 500 MHz, for eight cycles the clock counts will be                     N        clock            =                        (                      500            ×                                          10                6                            /              sec                                )                ⁢                                   ⁢                  (                      8            ⁢                                                   ⁢            cycles                    )                            20      ×                                    10            6                    /          sec                /        cycle                        N      clock        =    200  
The maximum count error is {fraction (1/200)}, which is much less than the errors that may result due to noise. It should be apparent that the signal quality is the first concern in achieving accurate measurements. After the signal is high-pass filtered, the time between zero crossings on the positive slope of the signal is measured to determine the frequency. The velocity is then the product of the frequency and fringe spacing. 
The velocity of the object is then determined by monitoring the frequency, which can be related to the surface velocity using geometric factors. This type of sensor is valuable since there is no physical contact with the moving surface, and the measurement is independent from the surface finish or color (assuming it is not a perfectly smooth surface without Surface scratches or small features). However, systems for making LDV measurements are often quite expensive because they require high precision optical components.
Therefore, there is a need for an inexpensive LDV system that still offers accurate and precise measurements of the velocity of moving objects.